Mouthwash may seem a beneficial, or at least harmless, addition to a daily tooth brushing routine.

But a new study suggests that swilling with anti-bacterial fluid could be killing helpful microbes, which live in the mouth and protect against obesity and diabetes.

The research was published in the journal Nitric Oxide.

While mouthwash is supposed to target the bacteria, which cause plaque and bad breath, in fact, it is indiscriminate, washing away beneficial strains.

Researchers at Harvard University found that people who used mouthwash twice a day were around 55 per cent more likely to develop diabetes or dangerous blood sugar spikes, within three years.

Although previous studies have found that poor oral hygiene can lead to health problems elsewhere in the body, it is the first research to show that seemingly positive practices can have unexpectedly negative consequences.

Kaumudi Joshipura, professor of epidemiology at Harvard School of Public Health, said: “Most of these antibacterial ingredients in mouthwash are not selective.

“In other words, they do not target specific oral bacteria-instead, these ingredients can act on a broad range of bacteria.”

The study looked at 1,206 overweight people aged between 40 and 65 who were deemed at risk of getting diabetes.

Over the study period around 17 per cent of people developed diabetes or pre-diabetes, but that rose to 20 per cent for those using mouthwash once a day, and 30 per cent for those who used it in the morning and evening.

Prof Joshipura said helpful bacteria in the mouth can protect against diabetes and obesity, including microbes which help the body produce nitric oxide, which regulates insulin levels.

Nitric oxide is also important for regulating the metabolism, balancing energy and keeping blood sugar levels in check.

Mouthwashes date back thousands of years, but the first commercial product was developed in the late 19th century, and named Listerine, after Joseph Lister, the British surgeon. It was originally used as a surgical antiseptic but by the 1920s was being sold as a cure for bad breath, as well as a floor cleaner and dandruff remedy.

Yet getting rid of helpful bacteria also makes room for potentially harmful bacteria to thrive, the researchers warn, and have advised that sticking to one rinse a day may be advisable.

The British Dental Associationdoes not list daily mouthwash use as a necessary component of proper oral health care and warn the fluid, does not ensure “food buildup and plaque are removed from your teeth.”

A study published earlier this year in the Journal of Periodontal Research found that some mouth bacteria seem to protect against diabetes and obesity.

And another study from 2013 found just a week of mouthwash could decrease a person’s oral nitrite production by 90 per cent, lowering blood nitrite levels by a quarter. These shifts in production led to visible blood pressure spikes.

“The indiscriminate routine use of antibacterial mouthwash products may cause more harm than good, in light of recent studies, and further supported by findings from this study,” the new research concluded.

“Mouthwash use may also have a detrimental impact on diabetes control and possible complications, as these share some common NO-mediated pathways with blood pressure and diabetes.”